The present invention relates to a fuel assembly and a fuel spacer and, more particularly, to the structure of a fuel assembly and a fuel spacer that is capable of promoting heat transfer from the fuel rods serving as the heat source to the coolant so as to raise the allowable power level of the fuel assembly, and also capable of lowering the void ratio so as to increase the reactivity.
Spacer structures directed to improvement in the heat transfer efficiency of the fuel assembly in a pressurized water reactor (PWR) are known from, for example, Japanese Patent Application No. 42-32372 corresponding to U.S. Pat. No. 3,395,077 and No. 3,379,619. In this example, the structure is such that, in the center of four sides surrounding fuel rods, a grating-type spacer and an obstacle serving as a vane are provided.
In this structure, a coolant flows over the peripheries of the fuel rods in such a manner as to cover them in accordance with the configuration of the vane and the position at which it is mounted. When a coolant flows in a fuel assembly in this way, it is possible to promote the transfer of heat, thereby raising the allowable power level of the core.
The above-described art relates to a PWR. If the spacer structure is used in a boiling water reactor (BWR), it is impossible to achieve the above-mentioned object. In such cases, a void occurs in the core of the BWR, and the coolant flows in a two-phase flow in the region downstream of the position where the void is present.
FIGS. 11 and 12 show the manner in which a coolant flows in a two-phase flow region during its flow through the space surrounded by fuel rods in a conventional structure. In the two-phase flow region, the liquid phase flows while either adhering as liquid films to the surfaces of fuel rods 1 or existing as liquid drops in the vapor. On the other hand, the vapor contains the liquid drops, and flows in the space surrounded by the fuel rods 1. If the above-described conventional fuel spacer is used in the two-phase flow region, the coolant forms a flow which proceeds along the peripheries of the fuel rods 1. Such a flow can, under centrifugal force, act to strip off the liquid films adhered to the fuel rods 1, thereby decreasing the amount of liquid adhered to the fuel rods 1 in the form of liquid films. As a result, there is a risk that parts of the fuel rods 1 may be deprived of liquid, thereby causing their dry-out. This leads to a drop in the output that brings the coolant to a boil, in other words, a drop in the critical output.
The conventional spacer structure entails another problem. Because an obstacle is provided, this inevitably increases the degree of pressure loss in both single-phase and two-phase flows. This means that the allowable power level of the core is lowered correspondingly.
It is considered that, in a region where the coolant forms a two-phase flow, or a gaseous-and-liquid phase flow, due to the formation of a void in the core of a BWR, if it is possible to increase the amount of liquid-film-forming flow which flows while adhering to the fuel rods, it is then possible to promote heat transfer and raise the allowable power level of the core.
On the basis of this theory, proposals to overcome the above-described problems have been made, for instance, in Japanese Patent Unexamined Publications Nos. 61-90085 and 1-132990. Among these, the publication No. 1-132990 shows a fuel assembly including fuel rods, and a grating-type fuel spacer with spiral members provided at individual intersections of the grating, the spiral members supporting the fuel rods. In this fuel assembly, the spiral members are positioned while surrounded by adjacent fuel rods, and guide cooling water flowing through the inter-rod spaces toward the fuel rods. With this structure, however, since the flow of the cooling water, which is made spiral by the spiral members, abuts directly on the fuel rods within the fuel spacer, there is a risk that the directly abutting flow may strip off the liquid films flowing on the fuel rods. If such is the case, a reduced amount of liquid adheres, in the form of films, to the fuel rods at downstream positions of the fuel spacer.
The above-described risk is eliminated in Japanese Patent Unexamined Publication 61-90085 where a spiral flow is generated in the space between cylindrical cells. This publication shows a fuel spacer formed by assembling together a multiplicity of cylindrical cells allowing the insertion therein of fuel rods. The spacer is provided with guide portions for guiding the flow of cooling water in the direction of the fuel rods. The guide portions are formed in the side surfaces of the cylindrical cells defining spaces which are each surrounded by adjacent cylindrical cells and through which cooling water flows.
This conventional structure is, however, not completely free from problems. Since guide grooves constituting the guide portions are formed from the lower ends to the upper ends of the individual cells, the level of pressure loss is still high. Furthermore, because these oblique guide grooves cannot be formed with ease, and because it is impossible to form in one process the guide grooves as well as upper- and lower-spring portions for holding the fuel rods in their predetermined positions, the formation of the guide grooves inevitably leads to an increase in production cost. Another problem is that since a part of each cylinder is bent at a sharp angle to form a guide groove, this part fails to exhibit sufficient strength.